At the present time there are known in the art various methods of complex separation of air with obtaining oxygen, nitrogen, argon, krypton, xenon and neon.
These methods are based on low-temperature processes of rectification, distillation, dephlagmation and adsorption, as well as on high-temperature processes of catalytic hydrogenation, adsorption. Most wedely used for air separation is a method of a low-temperature rectification which provides obtaining both the main products, viz. oxygen, nitrogen, and fractions for the following recovery of argon, krypton, xenon and neon.
Known in the art is a method of air separation with the aim of recovering argon (U.S. Pat. No. 4,575,388) by way of a low-temperature rectification, comprising:
separating air in a double rectifier incorporating a high-pressure column, a condenser and a low-pressure column with taking-out oxygen, nitrogen and a gaseous argon fraction with a large content of oxygen from the latter;
enriching said argon fraction in a rectifying column with obtaining crude argon containing not less than 2% by volume of oxygen and nitrogen.
The obtained crude argon is heated, compressed and divided into two flows one of which is directed for further purification, whereas a second one is used for establishing an argon circulation cycle.
With this in view, gaseous crude argon is heated, compressed up to a pressure sufficient for evaporating liquid oxygen in said condenser, cooled in a heat exchanger and liquefied by heat exchange with the liquid oxygen from the condenser. The liquefied argon is introduced as a reflux into the rectifying column.
Using the argon circulation cycle makes it possible to enhance an argon recovery factor from 0.66 to 0.75. However, its formation requires using special devices for supplying liquid crude argon into the rectifying column mounted at a considerable height. This affects the operational reliability of the plant and makes its automatic control circuit more complicated.
This method does not provide complete separation of oxygen from argon, which calls for special methods of purification since due to a small difference between boiling points of oxygen and argon the conventional method of low-temperature rectification is impracticable.
Known in the art is a method of air separation by a low-temperature rectification, which provides recovery of argon, oxygen and nitrogen (DE, B, 3,840,506).
The method includes compression, purification, cooling and separation of not less than 70% by volume of air in a high-pressure column of a double rectifier into liquid air enriched with oxygen, and liquid nitrogen. The liquid air and nitrogen obtained are directed into a low-pressure column to be separated into oxygen, nitrogen gas and a gaseous argon fraction.
The argon fraction is fed into a rectifying column to be separated into an argon-nitrogen fraction and a liquid fraction of high-boiling components containing 90% by volume of oxygen, 0.15% by volume of nitrogen and argon being the balance. The fraction of high-boiling components is returned to the low-pressure column.
The method contemplates delivery of a part of gaseous air into the low-pressure column which is one of the reasons for lowering at factor of argon recovery (argon yield) from the air to a value of 0.5.
A pressure in the rectifying column is maintained equal to a pressure in the low-pressure column which limits a condensating temperature of vapours in its condenser and accordingly limits a nitrogen content therein to 0.05% by volume. At the same time the sensitivity to changes in parameters of the process increases, its stability decreases, which also results in lowering the argon recovery factor.
To reduce concentration of oxygen in the argon-nitrogen fraction obtained in the rectifying column, the column has a considerable height. Therefore removal of the liquid fraction of high-boiling components therefrom with the following supply into the low-pressure column requires using special devices, e.g. pumps. Presence of such devices reduces the operational reliability of the plant and complicates the automatic control circuit thereof.